1. Field of the Invention
The present invention relates to a diagnostic ultrasound system, or more particularly, to a diagnostic ultrasound system having the ability to implement an imaging method referred to as "harmonic echo imaging" in which ultrasonic waves are scanned with an ultrasonic contrast medium injected into a patient body for the purpose of detection and quantitative assessment of perfusion of blood through a tissue.
2. Description of the Related Art
A diagnostic ultrasound system for producing an image signal through transmission and reception of ultrasonic waves has been employed in various ways owing to the noninvasiveness of ultrasonic waves in the past. Conventional diagnostic ultrasound systems produce tomographic images of a soft tissue of a living body by adopting ultrasonic pulse reflection imaging. This imaging method is noninvasive and produces a tomographic image of the tissue. Compared with other medical modalities such as diagnostic X-ray imaging, X-ray CT imaging, MRI, and diagnostic nuclear medicine imaging, the imaging method has many advantages: real-time display is possible, a compact and relatively inexpensive system can be constructed, patient exposure to X- rays or the like will not occur, and blood flow imaging is possible owing to ultrasonic Doppler imaging. The imaging method is therefore most suitable for diagnosis of the heart, abdomen, mammary gland, and urinary organs, and for diagnosis in obstetrics and gynecology. In particular, pulsation of the heart or motion of a fetus can be observed in real time through simple manipulation that is as simple as placing an ultrasonic probe on a patient's skin. Moreover, since patient exposure need not be cared about, screening can be carried out many times repeatedly. Furthermore, there is an advantage that a system can be moved to a bedside position for ready screening.
Under these circumstances, a new screening method for screening the heart or abdominal organs using ultrasonic waves has been developed in recent years. The method is such that ultrasonic waves are scanned with an ultrasonic contrast medium injected trans-venously, and a resultant image is used to evaluate the kinetics of a blood flow (Refer to, for example, U.S. Pat. No. 5,410,516 or Published Japanese Translation of PCT International Publication for Patent Application No. 4-501518. Since trans-venous injection of a contrast medium is less invasive than trans-arterial injection, the method for evaluating the kinetics of a blood flow is becoming popular. As an ultrasonic contrast medium, for example, a "5%-diluted human albumin with bubbles produced manually or using a sonicator" is known. The main component of the contrast medium is microscopic bubbles that act as a source of reflect ultrasonic waves. The larger the amount and concentration of injected contrast medium is, the larger the effect of contrast imaging is. However, since the bubbles are crushed due irradiation of ultrasonic waves, the time during which the effect of contrast imaging persists is shortened. Although a contrast medium characteristic of high persistency and high durability against sound pressure has been developed in recent years, the long-term persistence of the contrast medium in a human body predictably raises invasiveness.
A technique for detecting the presence or absence of blood flow in a diagnostic region by checking if luminance is intensified by a contrast medium has been adopted for the most fundamental diagnosis using an ultrasonic contrast medium. For more advanced diagnosis, a technique for acquiring information of a temporal change in spatial distribution of a contrast medium in a diagnostic region by detecting the spread of a change in luminance or the extent of intensification of luminance has been adopted. Also employed is a technique for obtaining information of a temporal change in spatial distribution by measuring a time required for an injected contrast medium to reach a region of interest (ROI) and a temporal change in luminance of echoes deriving from a contrast medium in a region of interest (time density curve (TDC)) or a maximum luminance. Processing of detecting a change in the level of ultrasonic echoes from a contrast medium which is necessary for those techniques is conventionally such that a change in the level of luminance of a B-mode image is checked merely visually, or image data of a plurality of frames are stored temporarily in a memory and then read in order to plot a histogram and thus quantitatively measure a change in level of echoes or plot a TDC.
In recent years, "harmonic echo imaging" has been devised as a technique for improving the effect of intensifying contrast echoes affected by an ultrasonic contrast medium. The harmonic echo imaging is based on the fact that microscopic bubbles of a contrast medium facilitate occurrence of an acoustic nonlinear phenomenon, that is, generation of reflected waves or echoes of nonlinear components or especially harmonics which are components other than the fundamental components of transmitted waves, and intended to distinguish in level echoes of harmonics from echoes emanating from an organ in a body that hardly generates harmonics. For example, reflected waves or echoes include fundamental components of transmitted waves and harmonics thereof stemming from a contrast medium. By displaying echoes remaining after the fundamental components are removed by a filter, the harmonics. That is, an image reflecting the extent of intensification due to the contrast medium can be obtained.
As mentioned above, using the harmonic echo imaging, once a relatively small amount of contract medium is administered, the presence or absence of the contrast medium in a region of interest, that is, the perfusion of blood, can be observed, and information helpful in diagnosis can be acquired.
However, for diagnosis based on harmonic echo imaging, when mere visualization based on the harmonic echo imaging is carried out, various drawbacks described below arise.
First, there is a problem concerning a diagnostic mode. Before administration (injection) of an ultrasonic contrast medium, a nonlinear effect exerted by an organ, vessel, or blood in the body is limited. Even if harmonic echo imaging (harmonic mode) is carried out in this state, almost no image information can be acquired. Therefore, when a region of interest is searched for and defined prior to administration of a contrast medium, the harmonic mode must be changed to a B-mode (normal B-mode) designed for interpreting a normal gray-scale tomographic image. Manipulation becomes time-consuming because of this mode change.
Secondly, there is a problem concerning the specification of a region to be observed. Assume that an ultrasonic contrast medium is administered and the magnitude of dying and shadowing by the contrast medium is observed. Since the effect of dying and shadowing decays gradually, the contour of an organ may not be observable. In this case, if a region to be observed becomes inconsistent with an object organ of observation on a tomographic image by accident, an observer may not be aware of the fact. Since this kind of accident is predicted, the results of diagnosis are naturally accompanied with a concern about reliability.
Thirdly, there is a problem attributable to microscopic bubbles that are the main component of an ultrasonic contrast medium. It is known that the microscopic bubbles are very delicate and vanish with irradiation of specific ultrasonic waves. As for this matter, sound pressure in the presence of ultrasonic waves capable of demolishing bubbles has been reported in, for example, a thesis entitled "In vitro detection of cavitation induced by a diagnostic ultrasound system" written by Christy K. Holland et al. (IEEE Trans. on Ultrason. Ferroelec. and Freq. Cont., Vol. 39, No. 1, January 1992). This report was verified by an experiment conducted by the present inventor. It has been confirmed that the sound pressure of a level usable in clinical practice may cause collapse of bubbles.
Furthermore, it is known that bubbles have a frequency characteristic dependent on the size (diameter) of an incident ultrasonic beam in the presence of which a sound pressure is detected. In particular, at a resonant frequency, bubbles vibrate with a large amplitude and eventually collapse or cavitate. If vanishing of bubbles due to irradiation of ultrasonic waves occurs at a rate that is higher than the speed of supplying a contrast medium (microscopic foams), echoes intensified by the contrast medium cannot be acquired. Consequently, proper values of physical quantities serving as conditions for transmission such as the sound pressure in the presence of transmitted ultrasonic waves, the frequency of the transmitted ultrasonic waves, the transmission rate thereof, the aperture of a probe for transmission, and the focus of the transmitted ultrasonic waves must be taken into account. However, prior arts have not taken such a point into consideration.